In the field of a magnetic disc, a 2 MB MF-2HD floppy disc using Co-modified iron oxide has been generally loaded in a personal computer. However, along with the increase in the amount of data to be dealt with, the capacity thereof has become insufficient and the increase of the capacity of the floppy disc has been demanded.
Magnetic layers comprising an iron oxide, a Co-modified iron oxide, CrO.sub.2, a ferromagnetic metal powder, or a hexagonal ferrite powder dispersed in a binder, which are coated on a support have been conventionally widely used in magnetic recording media. Ferromagnetic metal powders and hexagonal ferrite powders among these have been known to have excellent high density recording characteristics.
In the case of a disc, as high capacity discs using ferromagnetic metal powders which are excellent in high density recording characteristics, there are 10 MB MF-2TD and 21 MB MF-2SD, and as high capacity discs using hexagonal ferrite, there are 4 MB MF-2ED and 21 MB Floptical, however, any of these are not satisfactory with respect to capacities and properties. As is the circumstance, various attempts have been made to improve high density recording characteristics. The following fact as to the orientation of magnetic powders has been found recently during the course of the development.
It is important that the acicular ratio of the ferromagnetic powder itself is made high to realize a high coercive force by the anisotropy in configuration, and when the magnetic recording medium is a tape-like medium, the degree of magnetic orientation of the magnetic layer itself is heightened in the same direction with the running direction of the head, and when the medium is a rotating recording medium such as a floppy disc, as recording is digital recording, it is important to reduce the output fluctuation in a circumferential direction as far as possible rather than increase the output, therefore, the magnetic orientation in the magnetic layer should be random orientation not having anisotropy (the orientation ratio is made nearer to 1).
For example, the following methods have been suggested.
JP-A-6-36261 (the term "JP-A" as used herein means an "unexamined published Japanese patent application") discloses a recording medium comprising a lower nonmagnetic layer and a thin magnetic layer by using the ATOMM (Advanced Super Thin Layer & High Output Metal Media Technology) in which the orientation ratio in the in-plane and the vertical direction of 0.85 or more and the squareness ratio in the vertical direction of from 0.3 to 0.65 can be obtained by conducting non-orientation and oblique orientation while the layer is in wet state. This method provides a magnetic disc having a uniform and high output in a circumferential direction and excellent overwriting characteristics as compared with conventional discs produced by non-orientation processes, and the method for producing the same. However, the long axis length and the crystallite size of the ferromagnetic metal powders used in the examples of JP-A-6-36261 are 0.20 .mu.m and 195 .ANG., respectively, which are large.
U.S. Pat. No. 4,923,766 discloses a method of conducting random orientation immediately after the orientation in the vertical direction. The patent provides recording media exhibiting less mechanical orientation, high orientation ratio and excellent modulation by conducting random orientation in addition to the vertical orientation. In the examples thereof, metal magnetic powders of a large particle size having a long axis length of 0.25 .mu.m and a high acicular ratio of 10 are used. Magnetic powders of such the magnetic powder having comparatively large particle size and high acicular ratio are liable to be in rows in the plane. However, metal magnetic powders having a small long axis length and a small acicular ratio are not sufficient for orientation conditions for ensuring high S/N ratio which is important for high density recording. Further, the layer constitution comprises a single magnetic layer, which is insufficient for achieving high capacity of magnetic discs.
JP-A-63-171427 discloses a method of orientation comprising orienting ferromagnetic powders in a specific direction of a magnetic field, and then randomly orienting in a weak alternating current magnetic field in almost the orthogonal direction thereto. The magnetic powder used in the examples of JP-A-63-171427 is .gamma.-Fe.sub.2 O.sub.3, which is small in the amount of magnetization as compared with metal magnetic powders, therefore, satisfactory electromagnetic characteristics cannot be obtained, and is insufficient for obtaining a high S/N ratio as well.
JP-A-1-105328 discloses a method which comprises orienting magnetic powders in the first place in a transverse direction and then uniformly non-orientating the powders by a solenoid in an alternating current magnetic field. In the method of JP-A-1-105328, the in-plane orientation ratio is surely improved but it is not sufficient for obtaining a high S/N ratio. Further, the layer constitution in the examples of JP-A-1-105328 comprises a single magnetic layer using Fe-based metal magnetic powders, therefore, sufficient electromagnetic characteristics cannot be obtained.
JP-A-5-53009 discloses a method of random orientation which comprises putting a plurality of bar magnets for orientation in parallel at a distance with each other in the traveling direction of the support in such a manner that the magnetic poles of the adjacent magnets facing the support are different and arranged obliquely to the traveling direction of the support with facing converse directions alternately.
For realization of high density recording, it is important to make a particle size of a magnetic powder smaller. If a particle size is made small, the width of the particle is spontaneously limited and the length of the particle becomes inevitably short, i.e., the particle has a small acicular ratio.
For example, a problem has arisen during the development of a floppy disc having a high capacity in a recording/reproduction system requiring an areal recording density density of 0.15 G bit/inch.sup.2 or more such that noise increases in particular as a particle size of a magnetic powder decreases, and the noise becomes more conspicuous with heightening the coercive force.
However, efficient means for solving this problem could not been found.